yade-dev team mailing list archive

[Luc Sibille <luc.sibille@xxxxxxxxxxxxxxx>]

Hi Václav,

Just for information, when I was using SDEC, it was giving exactly the 
same result, absolutely no difference, when identical simulations where 
repeated (at least on the same computer).
SDEC was not using OpenMP, thus it may be an argument to discard the 2nd 
(Lunding's) explanation (?)

Best,
Luc

Le 05/03/2015 12:05, Václav Šmilauer a écrit :
> Hi there,
>
> I'm having an issue with results differing between initially identical 
> simulations. I know we have two stock explanations:
>
>   * the fairytale, authored by myself back then, that (force, ...)
>     summation order is non-deterministic due to OpenMP (threads
>     switching controlled by the kernel) and these small rounding
>     errors propagate in chaos-like fashion, having macroscopic effect
>     at the end;
>   * CPU may "randomly" keep some doubles in registers (which have more
>     width, IIRC 80bit?), and keep other ones in cache/RAM, which have
>     less width (64bit); thus some numbers will be randomly more
>     precise than others, which again leads to non-determinism and
>     propagation as above. I heard this from Stefan Luding (IIRC) some
>     10 years ago at a conference, but I am not sure how valid that is
>     at all; I have not heard about that from other sources.
>
> I am not conviced by either explanation, the scatter seems to be too 
> big (roller screens -- I get something like 2% scatter in resulting 
> PSD curves over a long simulation run multiple times, with completely 
> idetnical initial situation). Even though I use Woo, I assume the 
> problem is still the same as in Yade. So I would like to hear about:
>
>   * Someone else having that problem at all?
>   * Someone did a proper (quantitative) in-depth investigation of this?
>   * Someone run Yade through valgrind recently, to check for
>     uninitialized memory reads?
>   * I know that PFC 5.0 has what they call "deterministic mode"
>     simulations, someone knows the internals of what it does?
>     (Emanuele? If you may disclose that --)
>
> Cheers,
>
> Václav
>
>
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--
Luc Sibille
Université Joseph Fourier / IUT1 de Grenoble
Laboratoire 3SR UMR CNRS

Tel lab.: +33 (0)4 76 82 63 48
Tel IUT: +33 (0)4 76 82 53 36